This invention relates to novel printing plates, to a method for their preparation and to a lithographic printing process employing the plates.
Printing plates suitable for offset lithographic printing are known which comprise a support having non-image areas which are hydrophilic and image areas which are hydrophobic and ink-receptive.
The art of lithographic printing is based upon the immiscibility of oil and water, wherein the oily material or ink is preferentially retained by the image area and water or fountain solution is preferentially retained by the non-image area. When a suitably prepared surface is moistened with water and an ink is then applied the background or non-image area retains the water and repels the ink while the image area accepts the ink and repels the water. The ink on the image area is then transferred to the surface of a material upon which the image is to be reproduced, such as paper, cloth and the like. Commonly the ink is transferred to an intermediate material called the blanket which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
Ink-jetting is the non-impact method for producing images by the deposition of ink droplets on a substrate in response to digital signals. JP-A-53015905 describes the preparation of a printing plate by ink-jetting an alcohol-soluble resin in an organic solvent onto an aluminum printing plate. JP-A-56105960 describes the formation of a printing plate by ink-jetting onto a support e.g. an anodised aluminum plate an ink capable of forming an oleophilic image and containing a hardening substance such as epoxy-soybean oil together with benzoyl peroxide or a photo-hardening substance such as an unsaturated polyester.
European Patent Application No. 882584 describes a method of preparing a printing plate comprising producing an oleophilic image on the surface of a support by ink-jet printing the image on the surface using an aqueous solution or of a salt of a hydrophobic organic acid e.g. oleic acid. G.B. Patent Application No. 2,332,646 describes a method of preparing a printing plate comprising producing an oleophilic image on the surface of a support by ink-jet printing the image on the surface using an aqueous solution or aqueous colloidal dispersion of a polymer bearing water-solubilising groups wherein the water-solubilising groups interact with the surface of the support thereby binding the polymer to the support and rendering the polymer insoluble.
The prior art methods involve the use of organic solvents or photo-hardenable compounds or polymers which introduces some risk that the inlets may become blocked by the polymer.
A solution to these problems has now been invented in which an oligomer having hydrophilic and hydrophobic groups in the molecule, is used as an aqueous dispersion to prepare the image on the printing plate.
According to the present invention a method for the preparation of a lithographic printing plate comprises forming an oleophilic image on the surface of a hydrophilic support by depositing, preferably by ink-jetting, the image on the surface using an aqueous dispersion of an oligomer having in the molecule both hydrophilic and hydrophobic groups.
The method of the invention offers a rapid, simple and direct way to make a printing plate from digital data using relatively low cost equipment and without light sensitive materials.
Compared with the methods disclosed in the prior art, the method of the invention requires no processing of the plate and uses dilute aqueous dispersions having a low level of environmental impact and low health risk. In addition the risk of the jets being blocked by polymer is reduced.